When establishing a treatment plan in radiotherapy and the like, a perspective image of a target is generated virtually from three-dimensional volume data of the target (for example, the object to be treated) obtained beforehand. In the following, a perspective image of a virtual target generated from three-dimensional volume data will be called a virtual perspective image. A virtual perspective image is generally generated by adding voxel values of three-dimensional volume data existing on straight lines joining a viewing point with each virtual perspective image pixel.
A virtual perspective image is used for positioning in radiotherapy and the like. A virtual perspective image used for positioning is generated from three-dimensional volume data of the target used when a radiotherapy plan was established, in the same manner as when the treatment plan is established. Positioning is done by comparing the generated virtual perspective image with the perspective image captured at the time of treatment. The appearances of the virtual perspective image and the perspective image are sometimes different. Because the difference in appearance affects the positioning accuracy, the appearance of the virtual perspective image is controlled.
However, the appearance of the perspective image varies in accordance with imaging parameters such as the X-ray tube voltage and tube current in an apparatus (for example, X-ray imaging apparatus) used to obtain the perspective image. For this reason, in some cases, there can be a significant difference in appearance between the virtual perspective image and the perspective image.